Device designed for collecting solid debris in an electrolysis cell for the production of aluminium

ABSTRACT

Bucket shovel type collection unit, used in an aluminum production cell, comprising:
         a) a means of fixing onto a mobile support;   b) a connection actuated by a first actuator moving in relation to said mobile support in the vertical direction;   c) a frame interdependent of said connection;   d) at least one articulated bucket, swiveling around an axis assembled on said frame, having a blade and swiveling around said substantially horizontal axis.       

     Said first actuator is connected to a programmable control system able to determine the altitude of said axis and the difference in altitude between said blade and said axis, define the vertical movement to be imposed on said collection unit, and define and transmit to said first actuator the appropriate control flow to impose said vertical movement. The control flow may be control current transmitted by a variable speed transmission to an electrical motor or an oil flow feeding the chamber of a hydraulic actuating cylinder, controlled by a servo-distributor.

The invention relates to aluminum production using igneous electrolysis by means of the Hall-Héroult process. It more particularly relates to a device designed to collect solid debris immersed in, or floating upon, the electrolysis bath, and the molten metal, in particular mud coming from the electrolytic bath which accumulates on the bottom of the tank, as well as fragments of carbon and crust debris which occur in particular as a result of the various operations carried out before and during the removal of worn anodes.

Aluminum is produced industrially by igneous electrolysis, using the well-known Hall-Héroult process, in electrolysis cells. The plants contain a great number of electrolysis cells laid out in line, in buildings called electrolysis halls or rooms, and electrically connected in series using connecting conductors, in order to make the best use of the floor area of the plants. The cells are generally laid out so as to form two or more parallel lines which are electrically linked to each other by end conductors. In each cell, the electrolyte bath and the molten metal are contained in a tank, called an “electrolysis tank”, comprising a steel container, which is coated on the inside with refractory and/or insulating materials, and a cathode assembly located at the bottom of the tank. Anodes, typically made of carbonaceous material, are partially immersed in the electrolyte bath.

When running, an electrolysis plant requires work on the electrolysis cells, including replacement of worn anodes by new ones, sampling of molten metal and sampling or top-ups of electrolyte. In order to carry out this work, plants are generally equipped with one or more service units including an overhead traveling crane which can be relocated above and along the series of electrolysis cells, and one or more service modules each comprising a carriage able to be moved on the overhead traveling crane, and handling and servicing devices such as shovels and hoists, commonly called “tools”. These service units are often called “Pot Tending Assemblies” (PTA) or “Pot Tending Machines” (PTM). The service module generally includes, attached to the carriage, a rotary frame, called a tool-holder turret, which is able to turn around a vertical axis and is interdependent with said tools. Each tool may be fixed to the end of a cable operated by a winch attached to said turret, or to the end of an arm, which may be telescopic and/or articulated.

One of the operations necessary during anode replacement is cleaning the zone which was occupied by the worn anode and which is to be occupied by the new anode. This zone is primarily made up of the bath and the molten metal but may contain a number of solid fragments which must be removed before fitting the new anode. During electrolysis, a hard crust of fluorinated cryolite and alumina forms on the upper surface of the bath. This crust has the advantage of storing the heat within the bath and so constitutes an effective heat-insulating envelope. But it is extremely hard and adheres to the wall of the anode block, so that it proves to be necessary to break it around the worn anode, in order to allow the latter to be extracted. Typically, the crust is broken using tools such as tappers, called “crust-breakers”. During removal of the used anode, there is then an opening in the crust, which is left vacant until the new anode is fitted and which we will thereafter refer to as the “anode hole”. Breaking the crust and handling the worn anode block inevitably lead to the formation of solid pieces or parts which float upon, or remain in suspension in, the electrolysis bath, or fall to the bottom of the tank. It is then necessary to remove them by means of a collecting tool commonly called a “crust shovel”.

European patent application EP-A-0 440 488 describes an example of a crust shovel in conjunction with a special vehicle, distinct from the PTA. European patent application EP-A-0 618 313 describes, in not very great detail, an example of a PTA equipped with a device for breaking the crust in the vicinity of a worn anode as well as cleaning the anode hole. The crust shovel commonly used is a wrench made up of two buckets placed symmetrically in relation to a substantially vertical and articulated plane, swiveling around two substantially horizontal axes, which may be one and the same. Each bucket has a leading edge, also called a “blade”, opposite the leading edge of the other bucket. To collect the debris, the crust shovel is plunged in open position into the bath and is then moved from open to closed position by using at least one actuator which works either directly on a bucket, or preferably on a connecting rod assembly designed to make the buckets rotate in a substantially symmetrical direction in relation to each other, the solid debris located between the two buckets being trapped, while the liquid medium, a mixture of electrolyte bath and molten metal can still escape, in particular through openings made in the walls of the buckets.

Conventionally, the opening and closing movement of the crust shovel is driven by actuation of at least one pneumatic jack which acts on a connecting rod assembly designed to transform the translation movement of the jack into two symmetrical rotation movements of the buckets. Before placing the new anode in the cell, it must be checked that all crust and carbon debris in the anode hole has been removed. As some of this debris may be lying on the bottom of the tank, it is necessary to plunge the crust shovel into the liquid medium made up of the bath and the metal so that its leading edges barely touch the bottom of the tank. But as the leading edges of the buckets describe circular trajectories when the crust shovel closes to collect the debris, operating the shovel is very tricky, because the cathode assembly which makes up the bottom of the tank is likely to undergo significant damage during this operation To avoid such damage, the axis or axes of the buckets must be made to adopt a position at an altitude such that the leading edges of the buckets never touch the bottom of the tank during the operation, while being as close as possible to this bottom for efficient cleaning. However, this position is difficult to evaluate because there is no visual access to the bottom of tank. In addition, because of the circular trajectory described by the blades, this theoretical position makes the crust shovel ineffective in the phases where the blades are furthest away from the bottom of the tank, it being likely that some of the debris remaining on the bottom of tank may not be collected.

This anode hole cleaning operation using the crust shovel therefore encounters two antagonistic difficulties: either the shovel is too close to the bottom of tank and is likely to damage it, or it is too far away and cleaning is inadequate. Whatever procedure is adopted, there remains a considerable risk of general electric and magnetic instability in the operation of the tank, leading to a drop in the output of the plant.

European patent application EP-A-1 178 004 proposes a solution likely to solve the problem presented in the previous paragraph. This solution involves using a bucket shovel assembled on a vertical arm, but not fixing the frame interdependent with the axes of the buckets directly onto said vertical arm. For this purpose, the frame is duplicated in a part called the “shovel-holder frame”, which remains interdependent with the arm attached to the tool-holder turret and in a part called the “bucket-support frame”, vertically mobile in relation to the shovel-holder frame so that, as the center of instantaneous of rotation of the buckets may move whereas the arm remains motionless in relation to the bottom of tank, the leading edges of the buckets may be given a substantially rectilinear trajectory. The shovel may be placed so that its leading edges just touch the bottom of the tank throughout the shovel closing operation. However, such a solution makes the shovel mechanism much more complicated, with a complex connecting rod assembly for closing the buckets, including a load transmission rod, one end of which is articulated [ . . . ] on the buckets [ . . . ] and the other end of which is articulated on a rotary actuation rod, itself articulated on the bucket-support frame, said rotary rod being mechanically connected to the shovel-holder frame by means of a compensation rod articulated on the shovel-holder frame, said rotary rod being additionally made to rotate by means of an actuator jack, the point of application of which is interdependent with the bucket-support frame”. Such a solution makes it necessary to introduce into the crust shovel many adapters designed to function in a hostile environment and to undergo high amplitude vibrations, in particular because of the stresses associated with slamming the buckets. This involves frequent replacement of these parts that are prone to wear out quickly.

The goal set by the applicant was to achieve effective cleaning of the anode hole during anode replacement, without damaging the bottom of the tank, using a simple tool that is easy to clean and service and inexpensive to maintain.

A first subject according to the invention is a collection unit designed to collect the solid debris and mud in the liquid media of a cell for producing aluminum, such as the electrolysis bath and the molten metal, in particular a crust shovel designed for cleaning anode holes, comprising:

a) a means of fixing to fix said collection unit onto a mobile support able to move said collection unit above the zone to be cleaned;

b) a connection actuated by a first actuator which causes said connection to move in relation to said mobile support in the vertical direction;

c) a frame interdependent of said connection;

d) at least one articulated bucket, swiveling around a substantially horizontal axis, fitted to said frame, with a substantially horizontal blade actuated by a second actuator, causing said bucket to adopt a rotary movement around said substantially horizontal axis,

characterized in that said first actuator is connected to a programmable control system able to:

i) determine, directly or indirectly, the altitude of said substantially horizontal axis and the difference in altitude between said blade of said bucket and said substantially horizontal axis;

ii) define, from the values determined in i), the vertical movement which must be applied to said connection so that the altitude of said blade remains higher than a predetermined value;

iii) define and transmit to said first actuator a suitable control flow, so that the first actuator can apply said vertical movement to said connection.

Said first actuator makes it possible to move said connection vertically; this connection, typically in the form of a rigid rod, a sliding vertical mast, or a cable, is interdependent of the collection unit. According to the invention, the first actuator, also called the “lifting actuator”, in particular during the rotation of the buckets, is made to operate in such a way that the altitude of the bucket's axis of rotation is imposed according to that of the bucket blade. To achieve this, said first actuator is controlled during rotation of said bucket by said programmable control system which advantageously includes an instrumentation and control unit and a converter. The instrumentation and control unit:

-   -   acquires the data, typically provided by transmitters, relating         to said altitude of said substantially horizontal axis and said         difference in altitude between the bucket blade and the         substantially horizontal axis;     -   deduces from this, typically using a related computing memory, a         set point which must be imposed on the altitude of said         substantially horizontal axis so that the altitude of the blade         is at least equal to a predetermined value, which makes it         possible to avoid any risk of contact between said blade and an         obstacle located under said collection unit, for example the         tank bottom,     -   emits a signal representing said set point to the converter.

The converter translates said signal into a control flow and transmits said control flow to said first actuator. Depending on the type of actuator, the converter may, for example, be a servo-distributor in conjunction with a jack or speed controller in conjunction with an electric motor. In the first case, the control flow is an oil flow towards a jack chamber. In the second case, the control flow is an electrical signal or a control current, whose characteristic properties (current(s), frequency(ies), etc.) have an impact on the motor's direction and speed of rotation.

The altitude of the substantially horizontal axis and that of the blade can be measured directly by sensors but, because of the corrosive environment and the lack of accessibility, these direct measurements can advantageously be replaced by calculations made using indirect measurements. Said programmable control system may therefore be connected to a first sensor used to measure the vertical movement of said connection in relation to a reference level and to a second sensor used to measure, directly or indirectly, the difference in altitude between the bucket blade and the substantially horizontal axis around which the bucket swivels. The reference level may be a fixed level defined in the frame of reference of the electrolysis hall. It may also be connected to the mobile support on which the collection unit is fixed. In this latter case, it is obviously necessary to take into account a possible variation in altitude of said mobile support. With regard to the second sensor, a sensor used to determine the angular position of the bucket may be used. For this purpose, if said second actuator is a jack imposing a rotation on said bucket via a rod, an incremental position sensor measuring the movement of the stem of said jack in relation to the body of said jack may be used.

Advantageously, the characteristics of the control flow transmitted to the first actuator impact the direction and the intensity of the movement which said first actuator must carry out: the greater the variation noted between the measured altitude of the substantially horizontal axis and its set point altitude, the greater the intensity of the movement imposed on the actuator.

Obviously, it is possible to be more demanding as regards controlling the trajectory of the blade, since the efficiency of the collection unit decreases when the blade moves too far away from the bottom of the tank. In a preferred method of the invention, the set point to be imposed on the altitude. of said substantially horizontal axis is defined so that the altitude of the blade is not only greater, but also as close as possible to said predetermined value.

Advantageously, by making measurements a large number of times and by performing, at each measurement, a corrective movement of said connection so that the substantially horizontal axis is at the set point altitude that allows the blade to be at the desired altitude, a predetermined trajectory can be imposed on said blade. For this purpose, said instrumentation and control unit is advantageously an industrial Programmable Logic Controller (or PLC) which, on several occasions, preferably at regular time intervals, typically a few tens of milliseconds:

a) acquires the data supplied by said first sensor and said second sensor;

b) deduces from these data, using a computer program based on a kinematic model describing the trajectory of the blade in an appropriate reference frame, the set point which it is necessary to impose on the altitude of the bucket's pivotal axis, and

c) transmits to said converter a signal (S) representing said set point.

Advantageously, the computer program used in said programmable controller is based on a kinematic model which describes a trajectory of the blade passing above, but not too far from, the bottom of the tank. This trajectory can be deduced from the theoretical profile of the tank bottom by translation along a vector directed vertically upwards, the intensity of which corresponds to a pre-defined safety distance.

The collection unit according to the invention, may, for example, be a “crust shovel” used for cleaning the anode holes, comprising a frame and two buckets assembled on said frame, laid out symmetrically in relation to a substantially vertical plane and articulated, swiveling around two substantially horizontal axes, each bucket having a blade opposite the blade of the other bucket, the second actuator, interdependent of said frame, imposing on each of said buckets a substantially symmetrical rotation movement in relation to said substantially vertical plane, so that the solid debris located between the two buckets is trapped by said buckets.

Preferably, in order to impose a fluid movement on the bucket(s), the second actuator, also called “closing actuator” or “closing/opening actuator”, should not be a pneumatic jack, since this type of jack does not make it possible to constantly control the rotation speed of the bucket during the closing phase of the buckets. A(n) (electro)mechanical actuator may be chosen but, preferably, a hydraulic actuating cylinder should be chosen, supplied by a circuit of which a portion is assembled as a differential to endow the buckets with the slamming function described below, which is essential for the mechanical shovels used during anode hole cleaning.

To collect the debris, the collection unit, which is a crust shovel, is plunged into the bath while it is in open position; it is then moved from an open position to a closed position, using the closing actuator which acts on a connecting rod assembly designed to put the buckets into a substantially symmetrical rotation movement in relation to each other. The solid debris located between the two buckets is therefore trapped, whereas the liquid medium, a mix of electrolyte bath and of molten metal, can still escape, in particular through openings worked into the walls of the buckets. Part of this liquid medium, which is very viscous, adheres to the bucket walls, so that the buckets are covered with a gangue which must be removed each time the crust shovel goes into the tank, because the buckets become very quickly clogged and therefore inoperative. To remove a maximum amount of bath and metal, which cool and solidify, adhering to the surface of the buckets, an operation, called “bucket slamming” is performed. In this operation, the bucket closing/opening actuator is used so that the facing edges of the buckets are driven at such a speed that when they come into contact with each other a sufficiently violent shock ensues for the cooled bath and metal to come unstuck and be ejected from the surface of said buckets.

Traditionally, in particular because it was well suited to the slamming operation, the bucket closing/opening actuator consisted of one or more pneumatic jacks. Here, within the context of this invention, to ensure a fluid movement for the buckets and to better control the trajectory of the blades, it is advantageous to replace the pneumatic jacks by at least one double-acting hydraulic jack connected to a feed circuit which has at least two operating configurations for closing the buckets:

-   -   a first configuration, where the buckets move forward slowly but         where the closing jack can provide sufficient force to entrain         the solid debris encountered by the buckets,     -   a second configuration, in which it is not necessary to pass on         a force, but where it is necessary to pass on to the buckets         sufficient kinetic energy for the slamming function to be         carried out. This second configuration corresponds to a         differential assembly similar to that which is described below,         in example 1, in the comment on the rapid descent of the         collection unit, illustrated in FIG. 5. Obviously, as the         functions to be performed are different, the circuit supplying         the second actuator is different from that which supplies the         first actuator but the principle of the differential assembly         remains the same.

Previously, it was thought that effective slamming of the crust shovel could be performed only by pneumatic means. It was thought that, because of the compressed air, the buckets could much more easily and quickly be made to take on sufficiently quick symmetrical rotation movements for the shock resulting from this to detach and eject the bath and metal being solidified on the surface of the buckets. In addition, the source of compressed air was already available on the pot tending machine. Finally, it was desired to avoid setting up hydraulic circuits in places which may be in the immediate vicinity of the electrolysis bath.

Here, within the framework of this preferred method of the invention, one or more hydraulic actuating cylinders are chosen as the second actuator. To prevent it (them) from being located near the electrolysis bath, said hydraulic actuating cylinder(s) may either be moved away and an intermediate part acting on the connecting rod assembly be used, or they may be left near the buckets but protected from projections. The hydro-electric unit which is on board the pot tending machine and which is already high up so that it is far from the electrolysis bath may, for example, be used, and the hydraulic circuit necessary for operating the jacks be fitted so that the part most exposed to the hostile environment is restricted to the hoses which supply the compartments of the double-acting jack. To improve the protection of this portion of circuit still further with respect to the hostile atmosphere and possible projections of bath, the shovel-holder frame can be used as a screen, fitting the jacks above it and completing the protection of the hydraulic circuit hoses in the vicinity of said jacks by vertical walls surrounding said jacks and said hoses.

In addition, the fact of choosing one or more hydraulic jack(s) as the closing actuator makes it possible to devote the compressed air provided by the on-board compressor of the pot tending machine to other functions or, preferably, to choose a compressor of lower capacity, which would therefore be lighter, to equip said pot tending machine.

In a first embodiment according to the invention, said first actuator, also called “lifting actuator”, includes an electrical motor interdependent with said mobile support, a connection interdependent of said collection unit and coupled to said motor so that the rotation of said electrical motor causes said collection unit to move by means of said connection, and said converter is a variable speed transmission which transmits to said electrical motor a control current, whose characteristics allow said electrical motor to impose said vertical movement on said connection. Said electrical motor may be the engine of an electric jack, said connection being the rod of the jack bearing, or coupled with, a vertical mast supporting said collection unit. It may also be the motor of an electric winch, said connection being the cable supporting said collection unit. Example 2 described below illustrates such an embodiment.

In a second method of the invention, said first actuator includes at least one hydraulic actuating cylinder which includes a body interdependent with the mobile support and a piston connected to a rod which acts as said connection, and said converter is a distributor fitted to the portion of the hydraulic circuit which supplies the chamber on the rod side of said hydraulic actuating cylinder with a controlled flow. In this way, at least when said second actuator is activated, the volume of oil in the chamber on the rod side is imposed by said distributor which, advantageously, is an electro-hydraulic servo-distributor with controlled flow and controlled by said programmable control system. Preferably, said distributor is an electrically-controlled, proportional action, 4/3 servo-distributor.

In this second method of the invention said first actuator includes at least one hydraulic jack which makes it possible to move vertically said connection to which the rest of the collection unit is attached. Preferably, the handling arm of the collection unit is a telescopic arm, including a “mobile” mast sliding inside a “fixed” arm, the rod of said hydraulic actuating cylinder being interdependent with said “mobile” mast and the body of said hydraulic actuating cylinder being interdependent with said “fixed” mast, connected to said mobile support, for example a tool-holder turret fixed onto a carriage able to skirt round the beam of an overhead traveling crane, so that said collection unit can be moved and positioned above the working area before being taken down to the level of the anode hole. A “symmetrical” solution, involving making the mobile mast interdependent with the body of the jack and the rod interdependent with the “fixed” mast of the mobile support, is also possible. In both cases, the collection unit is made to rise by supplying the chamber on the rod side with oil. In this second method of the invention, a preferred embodiment includes a distributor controlled by a programmable controller which acquires at regular time intervals, typically a few tens of milliseconds, altitude H of the substantially horizontal axis around which the bucket swivels and value L of the movement of the piston rod of the second actuator, deduces from these values, using a related computer memory, the set point that it is necessary to impose on the altitude of the bucket's pivotal axis and injects a signal to said distributor in order to decrease or increase the volume of oil which supplies the chamber on the rod side and which is necessary to reach the right altitude.

Said first actuator may include several hydraulic jacks. However, as it is not easy to control the volume of oil in each chamber on the rod side, it is preferable to use as a first actuator a single jack whose chamber on the rod side is supplied using one only controlled distributor.

The device according to the invention in particular makes it possible to collect the debris while defining a safety distance between the tank bottom and the bucket blades: as soon as the estimated distance is lower than this safety distance, the control system sends to said distributor an instruction which increases the volume of oil in the chamber on the rod side of the jack in order to impose the desired altitude on the piston and consequently on the bucket's substantially horizontal axis of rotation.

Advantageously, for debris collection to be effective, the distance between the bucket blade and the tank bottom must also not be too great: as soon as the estimated distance is higher than a predetermined limiting distance, the control system sends an instruction to the distributor which decreases the volume of oil in the chamber on the rod side of the jack. In example 1 presented below, only the hydraulic circuit connecting the chamber on the rod side is acted upon: when it is necessary to move the bucket away from the tank bottom, the proportional action servo-distributor controls the oil flow sent at the required pressure into the chamber on the rod side; when it needs to be brought closer, it sends a controlled flow of oil to the tank from the chamber on the rod side, which is at a pressure corresponding substantially to the weight of the collection unit. In example 2, the variable speed transmission used emits a control current which can act not only on the amplitude of the rotation speed but also on the direction of rotation of the electric motor.

Preferably, the safety distance and the limiting distance are chosen to be as close as possible. As the tank bottom is generally a plane, this amounts to imposing a rectilinear trajectory on the leading edge of the bucket. Obviously, this trajectory can be defined more accurately, according to the actual geometry of the tank bottom at the place where fragment collection is to be carried out. In practice one defines a target altitude for the bucket blade and the programmable control system works in conjunction with a computing memory programmed to provide, according to the directly or indirectly measured opening angle of the bucket, the set point altitude which the instantaneous center of rotation of said bucket must have. The target altitude for the blade is constant if the tank bottom is considered to be a plane. It may be variable, according to the Position of the anode hole to be dealt with in the cell; this simply requires the use of a computing memory in conjunction with the more complex control system, giving different set points according to the place where the collection unit is working.

The device according to the invention therefore makes it possible to position the buckets as close as possible to the cathode, and therefore to increase the effectiveness of the debris collection operation without touching the tank bottom.

In practical terms, the collection unit, suspended from the mobile carriage which moves along the overhead traveling crane, is advantageously provided with an incremental position sensor which constantly gives the altitude of the bucket's (or buckets') substantially horizontal axis(es) of rotation. The movement sensor may, for example, be an encoder with cable or a laser rangefinder. The altitude of the tank bottom itself is known and can be checked regularly, for example by slowly lowering the collection unit placed in a predetermined position until the bucket blade touches the tank bottom. The bucket is in general bounded by an axial wall, i.e. a regulated surface generated by a generating line parallel to the pivotal axis and bearing on an open directing curve, and two transverse walls. These transverse walls have a substantially rectilinear edge, which joins the ends of the open curve. The position of the bucket can be characterized by the angle α which this edge forms with the vertical. By denoting as d the distance from this edge to the pivotal axis and as h the distance between the blade and the projection of the pivotal axis on said edge, the tilt angle α of the edge in relation to the vertical constantly gives the difference in altitude between the substantially horizontal axis and said blade, which is given by the expression: ΔZ=d cos α+h sin α. The tilt angle itself is directly related to a dimensional characteristic of the actuator which makes the bucket swivel. For example, for a jack, the tilt angle is directly related to the travel of the jack rod.

In a preferred embodiment of the invention, the programmable control system is an industrial PLC (Programmable Logic Controller) which, using a first sensor, acquires, at regular time as intervals, typically a few tens of milliseconds, the altitude of the pivotal axis of the bucket, and, using a second sensor, the value of the travel of the closing jack, and deduces from these values, using a computer program based on a kinematic model describing the trajectory of the blade in an appropriate frame of reference, the set point which it is necessary to impose on the altitude of the pivotal axis of the bucket and accordingly control the servo-distributor in order to add or remove the volume of oil necessary to reach the right altitude.

Advantageously, said hydraulic actuating cylinder is a double-acting jack, the rod of which is interdependent with the vertical mast and the collection unit , with a chamber on the rod side able to constantly impose on said rod a vertical movement upwards and a chamber on the piston side able to constantly impose on said rod a vertical movement downwards, it being possible to connect the two chambers, via at least one distributor, to a source of pressure or a tank, the feeding circuit including several portions of circuits which make it possible to provide the following hydraulic feed configurations:

a) a differential configuration, where the chamber on the rod side and the chamber on the piston side are connected to the pressure source, allowing the mast to descend at high speed;

b) a configuration corresponding to rest position, the collection unit remaining suspended, and the circuit arranged so that said collection unit can move vertically without effort should it encounter an obstacle;

c) a configuration in which the chamber on the rod side is connected to the pressure source, corresponding to the upward movement of the collection unit;

d) a configuration under controlled operation, in which the portion of circuit feeding the chamber on the rod side includes a distributor with controlled flow and controlled by a programmable control system including an instrumentation and control unit which acquires the data relating to the altitude of said substantially horizontal axis and the difference in altitude between the bucket blade and the substantially horizontal axis, deduces from these data the set point which it is necessary to impose on the altitude of said substantially horizontal axis and emits a signal representative of said set point to said distributor.

In example 1 given below, we describe these various operating phases of the collection unit in more detail.

Another subject according to the invention is a pot tending module designed to be used in a plant for the production of aluminum by igneous electrolysis and comprising a carriage and handling and servicing devices, characterized in that it also includes a collection unit according to the invention, as described previously.

Another subject according to the invention is a PTA for a plant for producing aluminum by igneous electrolysis including an overhead traveling crane and characterized in that it also includes at least one pot tending module according to the invention, as described previously.

Another subject according to the invention is the use of a pot tending module according to the invention for work on electrolysis cells designed for the production of aluminum by igneous electrolysis, in particular for cleaning the anode holes, in which said first actuator is controlled by said programmable control system so that said bucket blade(s) follow(s) a pre-defined trajectory, typically located above and parallel with the tank bottom.

Another subject according to the invention is a process for cleaning an anode hole during anode replacement, in which a collection unit according to the invention is used, said first actuator being assembled interdependently with a pot tending machine; one proceeds as follows:

a. using the PTA actuators, said collection unit is brought in closed position, in line with said anode hole, said first actuator being at rest;

b. said first actuator is actuated to descend quickly as far as a predetermined altitude, higher than the level of the bath located in the tank, in order to authorize opening of the collection unit;

c. said second actuator is actuated so that it opens the buckets until said buckets reach a reference open position, typically close to the maximum opening permitted by the travel of the second actuator;

d. said first actuator is actuated to descend slowly as far as a preset altitude; in a specific and preferred embodiment, the preset altitude targeted is the altitude reached by a point representing the connection when the bucket blade comes into contact with the tank bottom.

Said first actuator is actuated to descend slowly until contact of at least one blade on the tank bottom is detected; for example, if the actuator is a hydraulic actuating cylinder, a sensor to detect the moment when the pressure in the oil circuit which feeds the chamber on the piston side increases suddenly, a sensor giving the opening angle of the bucket and an incremental position sensor to give the altitude of the connection at the time of said contact are used;

e. from the altitude reached at the end of the previous stage, the height at which said blade must be is defined, taking a safety distance into account, and the trajectory that said blade must follow between said open reference position and the closed position is deduced;

f. said first actuator is actuated to raise said collection unit up to the origin point of the trajectory defined in the previous stage;

g. said second actuator is actuated, said first actuator being in controlled mode so that the blade follows the trajectory defined in e);

h. once the collection unit is closed, the first actuator is actuated in raise mode; then when the collection unit has reached a certain altitude, the PTA actuators are used to move the unit towards a zone which receives the collected debris.

FIG. 1 is a schematic cross-sectional view of a PTA in a typical electrolysis hall for the production of aluminum.

FIG. 2 illustrates a particular embodiment of a collection unit, which is a crust shovel, fitted to a telescopic vertical guide mast actuated by a hydraulic actuating cylinder.

FIG. 3 is a perspective view of the connecting rod assembly and bucket shovel of the embodiment shown in FIG. 2.

FIGS. 4 to 7 show four different configurations of a hydraulic circuit feeding the first actuator (lifting jack) for a collection unit according to the invention. These configurations correspond to the following operating processes: at rest (FIG. 4), fast descent (FIG. 5), rising (FIG. 6) and controlled operation (FIG. 7).

FIG. 8 schematically illustrates a collection unit according to the invention, in which the first actuator is an electromechanical jack.

Electrolysis plants for the production of aluminum include a liquid aluminum production area containing one or more electrolysis halls. The electrolysis hall (1) illustrated in FIG. 1 comprises electrolysis cells (2) and a PTA (5). The electrolysis cells (2) are normally laid out in row or files, each row or file typically comprising over a hundred cells. The cells (2) are laid out so as to leave an aisle throughout the length of the electrolysis hall (1). Cells (2) include a series of anodes (3) provided with a metal rod (4) for fixing the anodes and connecting them electrically to a metal anode frame (not shown).

The PTA (5) is used to carry out operations on the cells (2) such as changing anodes or filling the feed hoppers with crushed melt and aluminum fluoride (AlF3). It can be also used to handle various loads, such as tank parts, ladles of melt used in casting, or anodes. It may also be used to clean the anode hole, after the removal of a worn anode and before fitting a new anode.

The PTA (5) includes a overhead traveling crane (6) which may be moved above the electrolysis cells (2), and at least one pot tending module (7) including a mobile carriage (8), called a “tool holder”, that can be moved on the overhead traveling crane (6) and equipped with several handling and servicing devices (10), such as tools, one of which may be a crust shovel (100′). The tools are here fitted onto vertical telescopic masts (9) attached to the mobile carriage (8). As we have already seen, for example in patent application European EP-A 0 440 488, a crust shovel may also be moved and operated from a vehicle other than a PTA. The invention applies to any collection unit, no matter how it moves and positions itself above the working area.

FIGS. 2 and 3 illustrate a particular embodiment of a collection unit (100), which is a crust shovel (100′) fixed to the end of a telescopic arm, at the end of the mobile arm, here called a “shovel stem” (11). The shovel stem is a mobile vertical mast (9″) sliding inside a vertical mast (9′), which itself moves vertically, when driven by an actuator (not shown) while remaining interdependent with the tool-holder turret of the mobile carriage (8) of a pot rending module (7). The crust shovel includes a frame (110) provided with two buckets (120 a and 120 b) placed opposite each other, substantially symmetrically in relation to a substantially vertical plane and articulated, swiveling around two substantially horizontal axes (115 a and 115 b). Each bucket (120 a, 120 b) has a leading edge or blade (128 a, 128 b), opposite the blade (128 b, 128 a) of the other bucket (120 b, 120 a). The second actuator is here shown as two jacks (200, 201) interdependent with frame (110), functioning simultaneously, causing each bucket, via a connecting rod (300, 300′), to make a substantially symmetrical rotary movement in relation to the substantially vertical plane, so that the solid debris located between the two buckets is trapped by said buckets. Until the present invention, the two jacks of the second actuator were pneumatic jacks particularly well suited to the slamming operation.

EXAMPLES OF EMBODIMENTS Example 1 (FIGS. 2 to 7)

FIGS. 4 to 7 show four different configurations of a hydraulic circuit feeding the first actuator (50) for a collection unit according to the invention, which additionally has the characteristics described above (FIGS. 2 and 3).

The first actuator (50), or lifting jack, is a double-acting jack (51) with a body (55) and a piston (56) associated with a rod (52). The rod (52) is interdependent with the collection unit (not shown in FIGS. 4 to 7). The double-acting jack (51) has a chamber on the rod side (53), known as the lower chamber, able at any time to make the mobile vertical mast (9″) move vertically upwards, and a chamber on the piston side (54), known as the higher chamber, able at any time to make said mobile vertical mast move vertically downwards. The hydraulic circuit includes two portions (63) and (64) which feed the two chambers (53) and (54) of the double-acting jack (51). The circuit may be connected, via a three-position distributor, called the “direction distributor” (80), to the “pressure line” (P) and to the “return line” (R) of a hydraulic unit. The direction distributor (80) is naturally in rest position (802) and may be excited to be placed in one of the two other possible positions: position (803) in which rod (52) of the jack moves the collection unit down, and position (801) in which the jack rod moves said collection unit up.

The portion of circuit (64) includes a main branch (640) whose end is connected to the direction distributor (80) and whose other end is connected to the piston-chamber (54) of jack (51).

The portion of circuit (63) includes a main branch (630) one end of which is connected to the direction distributor (80) and the other end of which splits into two sub-branches, each of which is provided with a two-position distributor (81, 82), the first sub-branch (631 including 6310, 6311, 6312 et 6313) being associated with a shut-off valve (90), the second sub-branch (632 including 6320, 6321 and 6322) with the electro-hydraulic servo-distributor (83). The two sub-branches meet at their other ends to make up the portion of circuit (633), which feeds the rod-chamber (53) of jack (51).

FIG. 4 illustrates the circuit when the lifting jack is at rest. The direction distributor (80) is naturally in position (802), which connects the two portions of circuit (63) and (64) to each other by means of their respective main branches (630) and (640). The distributor (82) is in position (821) which stops circulation in the second sub-branch. Isolated by distributor (82) in position (821) and by shut-off valve (90) which is non-passing (the control pressures of connections (92) and (93) are insufficient to make it passing), the chamber-rod (53) is maintained, provided it does not receive any impact, at a substantially constant pressure, associated with the Weight of the collection unit. The branch of circuit (633) is provided with a safety device, integrated into the function of the shut-off valve (90), to limit the pressure in the rod-chamber in the event of an impact.

FIG. 5 illustrates the circuit when the hydraulic jack is moving down quickly. The direction distributor (80) is excited to occupy position (803), which makes the two portions of circuit (63) and (64) communicate with the pressure line (P) of the hydraulic unit, the two portions of circuit (63) and (64) also communicating with each other, by means of their respective main branches (630) and (640), at the level of the direction distributor (80) when it is in this position (803). The distributor (82) is in position (821) which stops circulation in the second sub-branch. Distributor (81) is in position (811) and allows the shut-off valve (90) to operate: whenever the resultant of the forces due to the control pressures from branch (92) and branch (93) is higher than a certain value, the shut-off valve (90) becomes passing.

Shut-off valve (90) is set at a critical value, typically around 180 bar, so that, as soon as its controls have sufficient pressures, it becomes passing and oil can run out of rod-chamber (53) towards piston-chamber (54), via branches (630) and (640), which communicate with each other at the level of the direction distributor (80), placed in position (803). In this way, the flow of oil from the hydraulic unit is increased by the flow of oil from the piston-chamber. If x is the ratio between the section of piston-chamber (54) and the section of rod (52), the flow from the hydraulic unit is multiplied by x, so that with such a differential assembly, the piston rod can descend at a speed x times faster than with a conventional assembly.

FIG. 6 illustrates the circuit when the hydraulic jack raises rod (52). The direction distributor (80) is excited to occupy position (801), which connects the main branch (630) with the pressure line (P) of the hydraulic unit, and makes the main branch (640) communicate with the tank of the hydraulic unit, via the return line (R). Distributor (82) is in position (821) and distributor (81) is in position (811). Oil under pressure goes through the main branch (630), passes through the distributor (81) in position (811) and joins portions (6313″) and (633) via the check valve (91), to feed the rod-chamber (53). As the piston rises, the oil in the piston-chamber (54) is sent out to the return line (R) of the hydraulic unit, via the main branch (640).

FIG. 7 illustrates the circuit when the lifting jack is activated in a controlled mode during fragment collection The direction distributor (80) is excited to occupy position (801), which connects the main branch (630) with the pressure line (P) of the hydraulic unit, and the main branch (640) with the tank of the hydraulic unit, via the return line (R) of the hydraulic unit. Distributor (82) is in position (822) and distributor (81) is in position (812). Depending on whether one wants to raise or lower the jack, the servo-distributor is activated so as to move into position (831) for lowering or (833) for raising.

The servo-distributor is controlled by a PLC (84) which receives the indications provided by two sensors:

-   -   the first indicates the distance (H) between a horizontal         reference level (N) (for example a platform of the tool-holder         turret) and a fixed point on the frame (110′) of the collection         unit (shown in FIG. 7 by the common horizontal level of the         buckets' axes of rotation), which gives the altitude of axis         (115′) around which the bucket (120′), the shape of which is         shown in dotted lines, swivels;     -   the second indicates the travel p of the closing jack rod         (200′), which, via a common rod (300′), orders the opening and         closing of the two buckets.

To any value L of the piston travel of the closing jack corresponds a given bucket opening angle, equal to twice angle (α) that the edge of the bucket (129′) makes with the vertical.

Denoting as d the distance from this edge (129′) to the pivotal axis (115′) and as h the distance between the blade (128′) and the orthogonal projection of the pivotal axis on said edge (129′), the difference in altitude between axis (115′) and said blade, given by the expression:

ΔZ=d cos α+h sin α

is constantly available

So to any position of the closing jack rod there corresponds an altitude at which said axis must be for blade (128′) to be at an altitude that is greater than or equal to a given value corresponding to the theoretical altitude of the bottom of the tank, with the addition of a certain safety margin, typically one or a few tens of millimeters. The programmable control system is a PLC (84) in conjunction with a computing memory (85) which enables it, as a function of the measurements of (H) and (L) transmitted, to define a set point altitude for the lifting jack (50). If the set point altitude is higher than the effective altitude of the lifting jack, there is a danger of collision between the bucket blade and the tank bottom, and the servo-distributor must be activated towards a position (833) to quickly correct the trajectory of said blade. The PLC (84) sends a signal (S) to the servo-distributor (83), which is a proportional action servo-distributor, imposing an oil flow (Φ) on the portion of circuit feeding the chamber on the rod side that increases in proportion to the difference with regard to the position of the set point. The signal has characteristics which make it possible to move the mobile part of the servo-distributor into a more or less advanced position of type (833), depending on the oil flow at the desired pressure, the oil coming from branches (630) and (632) and feeding the chamber on the rod side via branches (6321), (6322) and (633).

When, on the other hand, the altitude of the set point is lower than the effective altitude of the lifting jack (50), the servo-distributor must be activated to lower the altitude of the blade. The PLC (84) sends a signal (S) to the servo-distributor (83) to put it into a configuration corresponding to a position (831), where oil under pressure no longer feeds the chamber on the rod side, which is connected with the return line (R), via branches (65), (6321), (6322) and (633), the flow (Φ) draining off oil towards the return line being controlled by the opening of the servo-distributor, which is commanded by the signal emitted by the PLC.

At regular time intervals, typically a few tens of milliseconds, the PLC (84) acquires altitude H of the pivotal axis of the bucket and value L of the travel of the closing jack and, using a computer program based on a kinematic model which describes the trajectory of the blade in is an appropriate frame of reference, deduces from these values the set point which it is necessary to impose on the altitude of the pivotal axis of the bucket, and emits a signal (S) to the servo-distributor (83) in order to input or drain off the volume of oil necessary to reach the right altitude.

The second actuator (200′) is shown schematically here mainly to illustrate the role it plays in the operating principle of the first actuator (50), this being controlled in particular according to the spatial layout of said second actuator. This second actuator, of which the body is interdependent with the rod (52) of the first actuator (50), is here a double-acting jack connected to a feed circuit, part of which makes it possible to have a differential assembly for the slamming function.

To clean an anode hole during replacement of an anode, then, a collection unit according to the invention can be used, such as is illustrated in FIGS. 2 to 7, mounted on a vertical telescopic mast interdependent with an electrolysis pot tending machine (PTM). The procedure is as follows:

-   -   using the PTA actuators, said collection unit is brought, in         closed position, in line with said anode hole, said lifting jack         being at rest (the configuration illustrated in FIG. 4);     -   Said lifting jack is actuated to descend quickly (the         configuration illustrated in FIG. 5) as far as a predetermined         altitude, higher than the level of the bath located in the tank,         in order to authorize opening of the collection unit;     -   The bucket actuator known as the “closing jack” is actuated, so         that it opens the buckets until they reach a reference open         position, typically close to the maximum opening of the buckets         allowed by the travel of said closing jack.     -   The lifting jack is actuated to “slow” descent mode (specific         supply to the chamber on the piston side) until contact of at         least one blade is detected on the tank bottom; a sensor, for         example, is used to detect the moment when the pressure in the         oil circuit which feeds the chamber on the piston side increases         suddenly, and an incremental position sensor to record altitude         of the blade at the time of said contact;     -   From this altitude, the height at which said blade must be is         defined, taking a safety distance into account, and the         trajectory that said blade must follow between said open         reference position and the closed position is deduced;     -   Said lifting jack is actuated to raise said collection unit to         the point at the origin of the trajectory defined in the         previous stage (the configuration illustrated in FIG. 6);     -   The closing jack is actuated, with the lifting jack in         controlled mode so that the blade follows the trajectory defined         previously (the configuration illustrated in FIG. 7);     -   Once the collection unit is closed, the lifting jack is actuated         in raise mode (the configuration illustrated in FIG. 6); then         when the collection unit has reached a certain altitude, the PTA         actuators are used to move the unit towards a zone which         receives the collected debris.

Example 2 (FIG. 8)

FIG. 8 is a schematic view of a collection unit, in which the first actuator (50) is an electric motor (53′) powered using a circuit which makes it possible to control the rotation of said motor. Here, the electric motor is that of an electric jack (51′) which imposes a vertical movement on the connection (52′) interdependent with frame (110′). In a variant of this example, the electric jack is replaced by an electric winch, the connection then being a cable connected to said frame, the vertical movement of the latter, for example, being guided by a guiding device fixed to the mobile support.

Motor (53′), depending on the control current (I) sent by the variable speed transmission (83′), is constantly able to impose a vertical movement upwards or downwards on connection (52′) at the required speed. The variable speed transmission (83′) is controlled by a PLC (84) which receives the indications provided by two sensors:

-   -   the first indicates the distance (H) between a horizontal         reference level (N) (for example a platform of the tool-holder         turret) and a fixed point on the frame (110′) of the collection         unit (shown in the figure by the common horizontal level of the         buckets' axes of rotation), which gives the altitude of axis         (115′) around which the bucket (120′), the shape of which is         shown in dotted lines, swivels;     -   the second indicates the travel (L) of the closing jack rod         (200′), which, via a common rod (300′), orders the opening and         closing of the two buckets.

To any value L of the piston travel of the closing jack corresponds a given bucket opening angle, equal to twice angle (α) that the edge of the bucket (129′) makes with the vertical. Denoting as d the distance from this edge (129′) to the pivotal axis (115′) and h the distance between the blade (128′) and the orthogonal projection of the pivotal axis on said edge (129′), the difference in altitude between axis (115′) and said blade, given by the expression: ΔZ=d cos α+h is constantly available

So to any position of the closing jack rod there corresponds an altitude at which said axis must be for blade (128′) to be at an altitude that is greater than or equal to a given value corresponding to the theoretical altitude of the bottom of the tank, with the addition of a certain safety margin, typically one or a few tens of millimeters. The programmable control system includes a PLC (84) in conjunction with a computing memory (85) which enables it, as a function of the measurements of (H) and (L) transmitted, to define the set point altitude for the substantially horizontal axis around which a bucket swivels. If the set point altitude is higher than the effective altitude, there is danger of collision between the bucket blade and the tank bottom. The variable speed transmission (83′) is then activated in such a way that it can quickly correct the trajectory of the blade. The PLC (84) sends a signal (S) to the variable speed transmission (83′), which converts said signal into a control current (I) which imposes on said electric motor a direction of rotation and a speed that increases in proportion to the difference with regard to the position of the set point. Conversely, when the set point altitude is lower than the effective altitude, the variable speed transmission (83′) is activated in such a in way that it can quickly correct the motor control system, to lower the altitude of the blade. The PLC (84) sends a signal (S) to the variable speed transmission (83′), which imposes on the electric motor a direction of rotation and a speed that increases in proportion to the difference with regard to the position of the set point. 

1. A collection unit designed to collect solid debris and mud in liquid media of a cell for producing aluminum, comprising: a) a means of fixing to fix said collection unit onto a mobile support able to move said collection unit above a zone to be cleaned; b) a connection actuated by a first actuator which causes said connection to move in relation to said mobile support in a vertical direction; c) a frame interdependent of said connection; d) at least one articulated bucket, swiveling around a substantially horizontal axis, fitted to said frame, with a substantially horizontal blade, said bucket being actuated by a second actuator, interdependent with said frame, causing said bucket to adopt a rotary movement around said substantially horizontal axis, characterized in that said first actuator is connected to a programmable control system able to: i) determine, directly or indirectly, the altitude of said substantially horizontal axis and a difference in altitude between said blade of said bucket and said substantially horizontal axis; ii) define, from the altitude and the difference in altitude determined in i), a vertical movement which must be applied to said connection so that an altitude of said blade remains at least equal to a predetermined value; iii) define and transmit to said first actuator a suitable control flow, so that the first actuator can apply said vertical movement to said connection.
 2. Collection unit according to claim 1, characterized in that said programmable control system includes: a) an instrumentation and control unit which is configured to acquire data relating to said altitude of said substantially horizontal axis and said difference in altitude between the bucket blade and the substantially horizontal axis, deduce from the data a set point to be imposed on the altitude of said substantially horizontal axis so that the altitude of the blade is at least equal to a predetermined value, which makes it possible to avoid any risk of contact between said blade and an obstacle located below said collection unit, and emits a signal representative of said set point; b) a converter that is configured to translate said signal into a control flow and transmit said control flow to said first actuator.
 3. Collection unit according to claim 1 or 2, characterized in that said programmable control system is connected to a first sensor making it possible to measure the vertical movement of said connection in relation to a reference level and to a second sensor making it possible to directly or indirectly measure the difference in altitude between said blade of said bucket and said substantially horizontal axis.
 4. Collection unit according to claim 3, characterized in that said second actuator is a jack imposing a rotation on said bucket via a rod, and in that said second sensor is an incremental position sensor measuring movement of the rod of said jack in relation to the body of said jack.
 5. Collection unit according to claim 3, in which said instrumentation and control unit is an Programmable Logic Controller (or PLC) which is configured to: a) acquire the data provided by said first sensor and said second sensor at regular time intervals; b) deduce from the data, using a computer program based on a kinematic model describing the trajectory of the blade in an appropriate reference frame, the value of the set point which it is necessary to impose on the altitude of the bucket's pivotal axis, and c) transmit to said converter a signal representing said set point.
 6. Collection unit according to claim 5, in which said computer program implemented in said programmable controller is based on a kinematic model describing a trajectory of the blade which is deduced from the theoretical profile of the tank bottom by translation along a vector oriented vertically upwards, the intensity of which corresponds to a preset safety distance.
 7. Collection unit according to claim 1, characterized in that the collection unit includes a frame and two buckets assembled on said frame, laid out symmetrically in relation to a substantially vertical plane and articulated, swiveling around two substantially horizontal axes, each bucket having a blade opposite the blade of the other bucket, the second actuator, interdependent of said frame, imposing on each of said buckets a substantially symmetrical rotation movement in relation to said substantially vertical plane, so that the solid debris located between the two buckets is trapped by said buckets.
 8. Collection unit according to claim 1, characterized in that said second actuator is a hydraulic actuating cylinder fed by a circuit which has at least two operating configurations for closing the buckets: a) a first configuration, in which sufficient force is transmitted to the buckets to make it possible to entrain the solid debris encountered by the buckets, b) a second configuration, corresponding to a differential assembly, in which sufficient kinetic energy is transmitted to the buckets for the slamming function to be carried out.
 9. Collection unit according to claim 2, characterized in that said first actuator includes an electrical motor interdependent with said mobile support, a connection interdependent with said collection unit and coupled to said motor so that rotation of said electrical motor causes the movement of said collection unit via said connection, in that said converter is a variable speed transmission which transmits to said electrical motor a control current, whose characteristics allow the electric motor to impose said vertical movement via said connection.
 10. Collection unit according to claim 9, in which said electric motor is the motor of an electromechanical jack and in which said connection is a rod of said electromechanical jack.
 11. Collection unit according to claim 9, in which said electric motor is the motor of an electric winch and in which said connection is a cable of said winch.
 12. Collection unit according to claim 2, in which said first actuator includes a hydraulic actuating cylinder including a body interdependent with said mobile support, said connection being a rod of said hydraulic actuating cylinder, and in which said converter is a distributor mounted on a portion of the hydraulic circuit which supplies a chamber on a rod side of said hydraulic actuating cylinder with a controlled flow.
 13. Collection unit according to claim 12 in which said distributor is an electro-hydraulic servo-distributor with controlled flow to control a volume of oil inside the chamber on the rod side.
 14. Collection unit according to claim 12, in which said distributor is controlled by a Programmable Logic Controller which is configured to acquire, at regular time intervals, altitude H of the swiveling axis of the bucket and value L of the movement of the piston rod of the second actuator, deduce from these values, using a related computing memory, the set point which it is necessary to impose on the altitude of the pivotal axis of the bucket and inject a signal to said distributor in order to decrease or increase the volume of oil which feeds the chamber on the rod side and which is necessary to reach the right altitude.
 15. Collection unit according to claim 12, in which said hydraulic actuating cylinder is a double-acting jack, the rod of which is interdependent with said collection unit, with the chamber on the rod side able to constantly impose on said rod a vertical movement upwards and a chamber on the piston side able to constantly impose on said rod a vertical movement downwards, it being possible to connect the two chambers, via at least one distributor, to a source of pressure or a tank, the feeding circuit including several portions of circuits which make it possible to provide the following hydraulic feed configurations: a) a differential configuration, where the chamber on the rod side and the chamber on the piston side are connected to the pressure source, allowing the mast to descend at high speed; b) a configuration corresponding to rest position, the collection unit remaining suspended, and the circuit arranged so that said collection unit can move vertically without effort should said collection unit encounter an obstacle; c) a configuration in which the chamber on the rod side is connected to the pressure source, corresponding to the upward movement of the collection unit; d) a configuration under controlled operation, in which the portion of circuit feeding the chamber on the rod side includes a distributor with controlled flow and controlled by a programmable control system including an instrumentation and control unit which is configured to acquire the data relating to the altitude of said substantially horizontal axis and the difference in altitude between the bucket blade and the substantially horizontal axis, deduce from these data the set point which it is necessary to impose on the altitude of said substantially horizontal axis and emit a signal representative of said set point to said distributor.
 16. Pot tending module designed to be used in a plant for the production of aluminum by igneous electrolysis and comprising a carriage and handling and servicing devices, characterized in that it also includes a collection unit according to claim
 1. 17. Pot tending assembly for a plant producing aluminum by igneous electrolysis including an overhead travelling crane characterized in that it includes also at least one pot tending module according to claim
 16. 18. Use of a pot tending module according to claim 16 for work on electrolysis cells designed for the production of aluminum by igneous electrolysis, in which said first actuator is controlled by said programmable control system so that said substantially horizontal axis follows a pre-defined trajectory.
 19. Process for cleaning an anode hole during anode replacement, in which a collection unit according to claim 1 is used, said first actuator being assembled interdependently with a pot tending machine; the method comprising: a. using the first and second actuators, said collection unit is brought in closed position, in line with said anode hole, said first actuator being at rest; b. said first actuator is actuated to descend quickly as far as a predetermined altitude, higher than a level of a bath located in the tank, in order to authorize opening of the collection unit; c. said second actuator is actuated so that it opens the buckets until said buckets reach a reference open position, typically close to the maximum opening permitted by the travel of the second actuator; d. said first actuator is actuated in “slow” descent mode as far as a preset altitude; e. from the altitude reached at the end of the previous stage, a height at which said blade must be is defined, taking a safety distance into account, and a trajectory that said blade must follow between said open reference position and the closed position is deduced; f. said first actuator is actuated to raise said collection unit up to a point origin of the trajectory defined in the previous stage; g. said second actuator is actuated, said first actuator being in controlled mode so that the blade follows the trajectory defined in e); h. once the collection unit is closed, the first actuator is actuated in raise mode; then when the collection unit has reached a certain altitude, the first and second actuators are used to move the unit towards a zone which receives the collected debris. 